Hollow fibers having semi-permeable walls have been used extensively in recent years in fluid separation processes which separate dissolved components, or solutes from a fluid. Numerous devices which incorporate semi-permeable hollow fibers as the separating membrane have been employed commercially in reverse osmosis and ultrafiltration processes such, for example, as desalinating sea water, separating organic components from fluids, purification and concentration of fruit juices and other food products, etc.
Other devices have employed hollow fibers in industrial dialysis for purifying, separating or concentrating laboratory solutions and even more extensive use has occurred in purifying blood, in hemodialysis in a variety of artificial kidney devices.
All such devices require a multiplicity of hollow fibers, usually thousands of small diameter thin walled fibers, in which the permeable walls perform the separation in the fluid flowing inside, or outside the fibers. To enable the separation, previous devices have enclosed the separating portion of the hollow fibers within a fluid tight chamber which is sealed from an inlet and outlet chamber by a tubesheet or header member. The open ends of the fibers must communicate with the interiors of the inlet and outlet chambers and must be sealed from the separating chamber and from each other within the tubesheet.
Those skilled in the art of fluid separation devices experienced many difficulties and encountered various problems in forming tubesheets which successfully encapsulated and supported each fiber sufficiently to permit the ends to be cut, or shaved to expose an open end of each fiber. Early attempts were made to form tubesheets by dipping or pressure impregnation, with cements or polymeric compositions, of one end of a precut bundle of hollow fibers. Such bundles contained thousands of very small diameter hollow fibers spun from organic polymeric compositions in substantially parallel arrangement with the ends terminating in a common plane. In addition to the difficulty of avoiding rupture or crushing of the delicate thin walls of the fibers while holding them during tubesheet formation the problem of plugging the open fiber ends was encountered. The impregnating tubesheet material entered the bores of the hollow fine fibers and capillary attraction increased penetration of the interiors of the hollow fibers an unacceptable distance because the plugged fibers had to be cut off and discarded. Various attempts were made to avoid plugging including pre-filling the open ends with a fusible material such as wax, or dipping a bundle in a vertical position with a displacing fluid in the fiber bores during dipping, or gas purging through the fibers during the tubesheet cementing, etc.; these procedures were not commercially acceptable.
One commercially used method for forming tubesheets on hollow fibers in artificial kidney devices of the type disclosed in U.S. Pat. No. 2,972,349 and available commercially from Cordis Dow Corporation is disclosed in U.S. Pat. No. 3,442,002. According to the method of U.S. Pat. No. 3,442,002, continuous monofilament hollow fibers are wound into circular hanks consisting of a plurality of fibers; the hanks are flattened to form a bundle having loops of fibers forming the end portions; a plurality of hanks are collected and encased in a surrounding circular jacket which is substantially filled with the hanks of fibers but with the looped ends protruding; a tubesheet mold is placed on each end over the protruding looped fibers and the entire jacket and mold portions are placed in a centrifuge; tubesheet resin is introduced into the molds and the centrifugal force generated by the spinning forces the resin to penetrate the fiber bundle; after the resin solidifies, a transverse cut is made to remove the looped ends of the fibers and expose the open end of the fibers within the tubesheet. It is apparent that this process is complex, is wasteful of the hollow fibers embedded in the discarded end sections, and is not suited for automation to a continuous or semi-continuous operation.
It is therefore the principal object of this invention to provide a process for encapsulating fibers within a tubesheet which overcomes the problems of impregnating pre-cut bundles of hollow fibers and provides a simpler, less expensive alternative to the centrifugal impregnation process of U.S. Pat. No. 3,442,002.
Another important objective of this invention is to provide a semi-continuous, or continuous process of potting continuous tows of hollow fibers to form fiber bundle assemblies comprising a pair of tubesheets axially spaced apart and encapsulating the ends of the intervening tow of fibers.